The present invention relates to a rotary vacuum kiln for the treatment of solid particulate material under conditions of high temperature and under a high vacuum.
Solid particulate material must, at times, be treated under a vacuum at high temperatures in order to provide a desired product. In the manufacture of tantalum powders, for example, for use in capacitors, at one or more steps in processing, the powder is heat-treated in a vacuum furnace. Such treatment may be used to drive off residual impurities and to provide a flowable powder. A present processing system involves placement of a stack of trays containing tantalum powder into a vacuum furnace and heating the entire tray assembly. After a comparatively short heat treatment, in such a batchwise treatment, the entire tray assembly is cooled and a small amount of air is admitted until a layer of tantalum oxide has formed on the powder particle surfaces to prevent pyrophoric combustion of the powder upon subsequent exposure to air. Such a treatment is time- and energy-consuming and requires expensive equipment. Also, the fixed bed geometry of the treatment results in material near the outside of the bed being heated sooner and hotter than the material in the middle of the bed or tray stack. Heat transfer is also slow. In addition, since the material on the outside of the bed is heated more than that on the inside, uneven sintering can occur. A non-uniform product can result with various portions of the charge having different physical properties from others. If the material on the inside is not sufficiently sintered, the resultant product is fragile and a large proportion of this material turns to a dust during subsequent handling of the product. Such dust or fines must be recycled for reprocessing.
It is an object of the present invention to provide an apparatus for high temperature treatment of solid particulate material, while under a vacuum, by the use of a rotating kiln that will provide a more uniform heat-treated product.
It is another object of the present invention to provide a method for the continuous high temperature treatment of solid particulate material, such as tantalum powder, while under a vacuum, using a rotating kiln so as to provide a more uniform heat treated product.
A rotating vacuum kiln has a rotatable refractory metal cylindrical vessel that includes a cool inlet zone, a hot intermediate zone, and a cool exit zone. A gaseous exhaust conduit extends through an end wall of the cylindrical vessel through the cool exit zone and to the hot intermediate zone. A first series of inner radiation shields are provided in the cylindrical vessel at the hot intermediate zone adjacent to the cool inlet zone, and a second series of inner radiation shields are provided at the hot intermediate zone adjacent to the cool exit zone.
A first vacuum housing encloses a feed chute that directs solid particulate material to the cool inlet zone of the cylindrical vessel while under vacuum, while a second vacuum housing encloses a discharge chute for discharging treated material from the cylindrical housing while also under vacuum. Solid particulate material is moved through the rotating refractory metal cylindrical vessel by the use of screw flights attached to the inner surface of the vessel wall or by tilting the cylindrical vessel to allow flow by gravity.
The hot intermediate zone of the cylindrical vessel is indirectly heated by electric resistance heating bands which are provided, spaced from and along the hot intermediate zone, while outer radiation shields surround the heating bands and the cylindrical vessel along the hot intermediate zone. The use of the heating bands, radiation shields, and first and second series of inner radiation shields, concentrate the heat in the hot intermediate zone of the cylindrical vessel and shield the cool inlet zone, cool exit zone, and associated mechanical equipment, such as drive equipment and support equipment, from the high temperatures of the hot intermediate zone.
A method of heating a solid particulate material to high temperatures includes providing a rotating refractory metal cylindrical vessel having a cool inlet zone, hot intermediate zone and cool exit zone, with a first series of inner radiation shields at the hot intermediate zone adjacent the cool inlet zone and a second series of inner radiation shields at the hot intermediate zone adjacent the cool exit zone. Solid particulate material is moved through the rotating refractory metal cylindrical vessel while under a vacuum from the cool inlet zone and heated to a temperature of between about 1000xc2x0 to 1700xc2x0 C. in the hot intermediate zone and then discharged from the cool exit zone of the rotating refractory metal cylindrical vessel.